Feat: Serve images directly from Cloudflare R2 CDN

API responses now include file_url and thumbnail_url fields. When
S3_PUBLIC_BASE_URL is configured, these point to the CDN domain;
when unset, they fall back to the existing API proxy paths so local
dev requires no additional setup. UI updated to use response URL
fields directly instead of constructing proxy URLs client-side.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/tests/integration/test_upload.py

@@ -132,6 +132,10 @@ async def test_upload_returns_thumbnail_key(authed_client):
     assert "thumbnail_key" in body
     assert body["thumbnail_key"] is not None
     assert body["thumbnail_key"].endswith("-thumb")
+    assert "file_url" in body
+    assert body["file_url"].startswith("/api/v1/images/")
+    assert "thumbnail_url" in body
+    assert body["thumbnail_url"].startswith("/api/v1/images/")
 
 
 @pytest.mark.asyncio
@@ -172,3 +176,6 @@ async def test_upload_succeeds_when_thumbnail_fails(authed_client):
     assert response.status_code in (200, 201)
     body = response.json()
     assert body["thumbnail_key"] is None
+    assert "file_url" in body
+    assert body["file_url"].startswith("/api/v1/images/")
+    assert body["thumbnail_url"] is None